The invention relates to electrochemical cells, particularly alkaline cells, having metal current collectors.
Conventional alkaline electrochemical cells are formed of a cylindrical housing (casing). The housing is initially formed with an enlarged open end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the housing edge over an edge of the insulating plug and radially compressing the housing around the insulating plug to provide a tight seal. A portion of the cell housing forms the positive terminal.
The cell contents of a primary alkaline cell (Zn/MnO2 cells) typically contain an anode comprising zinc anode active material, alkaline electrolyte, a cathode comprising manganese dioxide cathode active material, and an electrolyte ion permeable separator, typically comprising a nonwoven material containing cellulosic fibers and polyvinylalcohol fibers. The anode active material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent holds the zinc particles in place and in contact with each other. A single conductive metal nail, known as the anode current collector, is typically inserted into the anode material in contact with the end cap which forms the cell""s negative terminal. The nail is conventionally inserted into the anode and located along the cell""s central longitudinal axis. The current collector can be welded to the end cap. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. Preferably there is no added mercury to the anode, that is, the anode is essentially mercury free, thus containing less than 50 parts mercury per million parts total cell weight. The cathode material is typically of manganese dioxide and may include small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight. Electrolytic MnO2 (EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods. EMD is typically manufactured from direct electrolysis of a bath of manganese sulfate and sulfuric acid.
In the cathodes of conventional Zn/MnO2 alkaline cells the manganese dioxide composition is typically between about 70 and 87 percent by weight. Particulate graphite and aqueous KOH solution can be added to the manganese dioxide to form a cathode mixture. Such mixtures form a moist solid mix which can be fully compacted into the cell housing using plungers or other such compacting devices forming a compacted solid cathode mass in contact with the cell casing. The cathode material can be preformed into the shape of disks forming annular rings inserted into the cell in stacked arrangement so that their outer surface abuts the inside surface of the cell housing. In such embodiment the cathode inside surface faces the cell""s interior. The cell housing typically functions as the cathode current collector. The cathode inside surface is normally smooth and uniform resulting in a cathode having a uniform and constant wall (annular) thickness along the cell""s length. The cathode disks can then be recompacted while in the casing. The separator can be placed so that it lines the inside surface of the cathode. The spearator can be inserted as a sheet over the cathode exposed surface or it can be sprayed or coated in liquid form onto the cathode and subsequently dried to form a film as described in commonly assigned copending patent application Ser. No. 09/280,367, filed Mar. 29, 1999, now abandoned. The anode mixture can be inserted into the central void space available within the cathode disksxe2x80x94the separator material being between the anode and cathode surfaces.
There are increasing commercial demands to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates, typically pulsed drain, of between about 0.5 and 2 Amp, more usually between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 Watt. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications.
However, use of the cell in high power application tends to increase polarization effects. Polarization limits the mobility of ion transport within the electrode active material and within the electrolyte. Such polarization can result in a significant portion of the anode and cathode material remaining undischarged, thereby reducing the cell""s actual capacity (mAmp-hours) or service life (hours). It is thus desirable to find ways of making the cell more amenable to high power application without significantly increasing polarization effects or otherwise adversely affecting cell performance.
Recent art describes an approach wherein the alkaline cell cathode has passages or tunnels therein for insertion of the anode material, as shown in U.S. Pat. No. 5,869,205. The tunnels are completely surrounded by cathode material which is lined with ion permeable separator material. In another approach the cathode disks appear to have spaced apart cutout or indented regions which run circumferentially along the cathode""s inside surface (surface facing the cell""s interior) as shown in International publication WO 00/01022. This results in a cathode wall thickness which varies along the cell""s length. Alkaline cell cathode disks having cutout or indented regions which appear to run longitudinally along the cathode""s inside surfaces are shown in U.S. Pat. No. 6,342,317, filed Jul. 21, 1999, commonly assigned with the present application. In such design the cathode wall thickness varies circumferentially. Alkaline cells having such indented surfaces provide greater interfacial surface area at the interface between anode and cathode than conventional cells. This reportedly reduces the average current density (mAmp/cm2) at the anode/cathode interface resulting in improved actual capacity (mAmp-hrs), particularly under high power application.
When conventional nail anode current collectors are used with such alkaline cells, however, the average distance between the nail and the cathode surface is typically less than it would e in conventional cells having cathodes of constant wall thickness. This has a tendency to increase the cell""s internal resistance during discharge and therefore can result in less than optimum performance.
Applicant has determined that the performance of alkaline cells having an annular cathode with a lobed, namely an indented (curvilinear) surface can be improved by extending a portion of the anode current collector into the indented area. This reduces the cell""s internal resistance, and improves performance extending further the cell""s service life, particularly under high power application.
It has also been determined that the performance of alkaline cells having annular cathodes can be improved by employing cathode current collectors which are inserted into the cathode. Such cathode current collectors can be in the form of continuous sheets or disks of electrically conductive material inserted into the cathode. The cathode current collectors are desirably oriented so that they are positioned radially, namely, perpendicular to the cell""s central longitudinal axis or positioned so that they run along the length of the cathode, that is, parallel to the cell""s longitudinal axis. The cathode current collectors have particular utility when the cathode is non uniform wall thickness or has an indented or lobed inside surface. The insertion of the cathode current collectors, preferably in the form of conductive sheets or disks improves the cell""s service life and energy output, particularly under high power application.
The cathode current collector can desirably be formed of carbon or graphite sheets, graphite sheets with carbon fibers therein, or stainless steel or nickel foil. The material in the form of sheets or panels of polygonal shape, typically rectangular, which can be oriented so that it located within the cathode and extends longitudinally along the cathode length. Alternatively, the cathode current collector can be in the configuration of a disk having an outer edge, a hollow central region and inner edge bounding the hollow region. A plurality of disks can be placed within the cathode in spaced apart arrangement along the length of the cathode. Each disk is preferably oriented so that it is perpendicular to the cell""s central longitudinal axis. The outer edge of the disk is preferably in contact with the cell housing. If cathodes having indented inside surface are employed the cathode current collector disks can have an inside edge which can likewise be indented. Thus, when the disk is inserted into the cathode, the indentations on the inner edge of cathode current collector disk will be aligned with the indentations on the cathode inside surface. The cathode current collectors of the invention can be employed independently of the anode current collectors of the invention or can be used in combination therewith. The use of the cathode current collectors of the invention increases the cell""s actual capacity and energy output, particularly under high power application.
In one aspect the cell is an alkaline cell having a cylindrical steel housing (casing), an anode comprising particulate zinc and a cathode comprising manganese dioxide, wherein the cathode runs annularly along the length of the inside surface of the housing, wherein the portion of the cathode""s inside surface (surface facing the cell""s interior) comprises at least one and preferably a plurality of surface indentations. The surface indentations are preferably longitudinal indentations between protruding surfaces or lobes running in the direction of the cell""s longitudinal axis, preferably parallel to the cell""s longitudinal axis. (The term longitudinal surface indentation as used herein and in the claims shall mean an indentation on the surface of a member, the indentation running at least substantially in the cell""s longitudinal direction, that is, in the direction of the cell""s length.)
When the cell is assembled the cathode lines the inside surface of the housing forming an annular cathode. Each of said indentations (hereinafter the cathode indentations) preferably runs longitudinally along at least a major portion of the length of the inside surface of the cathode. Each indentation is characterized by being formed of a continuous surface forming the indentation wall defining a lobed surface which can be lined with ion porous separator material. The spearator can be inserted as a sheet over the cathode exposed surface or it can be sprayed or dip coated in liquid form onto the cathode and subsequently dried to form a film as described in commonly assigned copending patent application Ser. No. 09/280,367, filed Mar. 29, 1999, now abandoned. The indentation wall bounded by protruding lobes can be bent, angled or curved and is not closed but rather defines a channel having an opening running along the indentation""s length. The indentation opening faces the cell""s interior and preferably runs parallel to the cell""s central longitudinal axis. When anode active material is inserted into the interior of the cell, there is a pathway from the central portion of the anode through said opening into the indentation channels. The outer portion of the anode material thus fills the indentation channels.
Each cathode surface indentation can itself be formed of flat or curved surface or a combination of flat and curved surface. Each cathode surface indentation is defined by a wall formed of a continual surface which itself can be formed, for example, of connected portions of arcuate surfaces having the same or different radii of curvature. A portion of the indentation wall, thus, can be concave and a portion can be convex when viewed from the cell""s central longitudinal axis. The convex portion can form a lobe protruding into the cell interior. The indentation wall itself is preferably formed of a symmetrical surface. If a plurality of indentations are employed, they are preferably of the same size and shape and symmetrically located around the cathode inside surface. It is preferable to locate each indentation so that it runs substantially in the cell""s longitudinal direction, preferably, parallel to the cell""s central longitudinal axis. The planar axis of symmetry of each indentation channel can intersect the cell""s central longitudinal axis. Each indentation channel can typically be oriented along the cathode inside surface so that the cell""s central longitudinal axis intersects and lies within the planar axis of symmetry of each channel.
The anode current collector of the invention is formed of a conductive member, typically of metal which has extended surfaces. The anode current collector of the invention is inserted within the anode material so that at least a portion of said extended surface reaches near or into the cathode indentation channel, yet without touching the indentation wall or the ion porous separator lining the indentation wall. A portion of the anode current collector can pass downwardly through the channel opening while being inserted. The anode current collector can be selected from a variety of known electrically conductive metals found to be useful as current collector materials, for example, brass, tin plated brass, bronze, copper or indium plated brass.
In one aspect the annular cathode has a pair of oppositely facing indentations (first and second indentations) on the cathode inside surface. Each indentation has a channel (first and second channels) running longitudinally along the length of the cathode inside surface and preferably parallel to the cell""s central longitudinal axis. Each indentation is defined by a indentation wall defining an indentation channel having an opening running along the indentation""s length allowing access to the indentation channel from the interior of the cell. There is a protruding surface (lobe) between each indentation. The anode current collector comprises a pair of elongated members (legs) which can be in the form of two or more parallel wires physically connected to each other by a welded or molded brace or other support member, desirably oriented perpendicular to the wires. Preferably the pair of elongated members can be formed of a wire or nail bent into U-shape. The anode current collector is inserted into the cell""s interior (anode) so that one of the elongated members is preferably located within one of the cathode indentation channels and the other is located in said oppositely facing cathode indentation channel when the indentation channels are as deep as shown om FIG. 6. It should be well recognized that if the indentation channels are shallower than as shown in FIG. 6, then the current collector elongated members can be placed adjacent but not necessarily within the indentation channels. The U-shape bend is desirably located at the end of current collector closest to the cell""s negative terminal end cap and can be welded to said end cap. Alternatively, the anode current collector can be in the form of a metal or conductive plate or sheet having one edge penetrating into the first cathode indentation channel and the opposite edge penetrating into the second indentation channel. The anode current collector sheet is also desirably placed so that it intersects the cell""s central longitudinal axis lies along the channel""s planar axis of symmetry.
In another aspect the annular cathode has a plurality of longitudinal indentations on the cathode inside surface. Each indentation has a channel running along the length of the cathode and preferably parallel to the cell""s central longitudinal axis. Each indentation is preferably, although not necessarily, of the same size and shape and symmetrically oriented around cathode inside surface. Each indentation is defined by a indentation wall lined with ion permeable separator material defining a channel running along the indentation""s length. There is a protruding surface portion (lobe) between each indentation. Each indentation is formed of a continuous wall surface having an opening which faces the cell""s interior and allows access into the indentation channel from the interior of the cell. Thus, when the cell is filled with anode active material, the anode material extends into said indentation channels. The anode current collector comprises a plurality of elongated members (legs) which can be in the form of parallel wires physically connected to each other by a welded or molded brace, typically oriented parallel or perpendicular to the wires depending on its design. The anode current collector is inserted into the cell""s interior (anode) so that one of the elongated members is located within one of the cathode indentation channels and another is located within another of the cathode indentation channel. Preferably, the anode current collector elongated members are in number equal to the number of cathode indentation channels, so there will be one current collector elongated member (leg) inserted into (or adjacent) each of the cathode indentation channels. The anode current collector is inserted into the cell""s interior so that each elongated member is positioned within a respective cathode indentation channel without contacting the indentation wall or ion porous separator lining the indentation wall. The end of the current collector closest to the cell""s negative terminal end cap can be welded to said end cap. Alternatively, the anode current collector can be in the form of a plurality of metal or conductive plates or sheets. The plates or sheets can intersect at a common line. The current collector can be inserted into the cell interior so that the outer longitudinal edge of each plate or sheet penetrates into a different one of the indentation channels. The anode current collector is desirably placed so that its central longitudinal axis is parallel to, preferably coincident with the cell""s central longitudinal axis. In such orientation each of the current collector plates or sheets cuts a path from the interior of the anode into the cathode indentation channel regions filled with anode material.
Alternatively, the anode current collector can have an antenna shape, that is, can be formed of a central longitudinal elongated member with a plurality of spaced apart (horizontal) elongated members connected perpendicularly to the central longitudinal member. The longitudinal member and spaced apart (horizontal) elongated members connected perpendicularly thereto can be in the form of metallic or conductive wires of the same or varying diameter. When the current collector is inserted into the cell""s anode, the exposed end of each spaced apart (horizontal) elongated member penetrates into (or is positioned adjacent) one of the cathode indentation channels. A plurality of the horizontal current collector members are preferably staggered along the length of the central longitudinal member so that when the anode current collector is inserted into the anode, there will be a plurality of horizontal current collector members with an end of each horizontal member penetrating into a respective cathode indentation channels.
The zinc/MnO2 alkaline cell of the invention is essentially mercury free, that is, does not contain any added mercury. Therefore, the cell of the invention has a total mercury content less than about 50 parts per million parts of total cell weight, preferably less than 20 parts per million of total cell weight, more preferably less than about 10 parts per million of total cell weight. The cell of the invention also preferably does not contain added amounts of lead and thus is essentially lead free, that is, the total lead content is less than 30 ppm, desirably less than 15 ppm of the total metal content of the anode.